The present invention relates to low voltage vacuum switches which are typically used in electrolytic chemical processing plants. The switches are used as shorting switches which carry thousands of amperes of current at low D.C. voltage levels. The switches are actuated for shunting a chemical cell such as described in U.S. Pat. No. 4,075,448.
A low voltage vacuum switch is described in abandoned application Ser. No. 650,322, filed Jan. 19, 1976 entitled "Low Voltage Vacuum Shorting Switch". The switch described in the copending application utilizes flexible annular corrugated annular diaphragm end walls, which are sealed to an annular insulating ring-like body portion at their outer perimeters, and to cylindrical conductive contacts at their interior perimeters to complete the evacuated switch. The flexible annular corrugated diaphragm members provide the requisite flexibility so that the contacts can be moved into and out of contact with axial force. These flexible end members are relatively thin metal members with several annular corrugations between the interior perimeter and the exterior perimeter. When the contacts are separated, a D.C. arc is struck between the separated contacts, and several thousand amperes of current will flow until interruption is effected when the arc voltage across the electrodes is greater than the applied voltage from the circuit to which it is connected. It has been found that during the dissipation of the inductive energy of the circuit during arcing the magnitude of the arc may be such that the arc moves off the contact base and is directed onto the thin annular diaphragm wall with a possibility of burning a hole in the diaphragm. This is particularly a problem with respect to the flexible annular diaphragm at the electrically positive connected contact of the switch.
It has also been observed that during the arcing process contact material which is evaporated can be thrown out from between the contacts and deposited along the annular insulator body. This evaporated contact material will then be deposited as a film across the insulator body and can give rise to high leakage currents between the contacts along this thin deposited film. Such a leakage current creates several problems associated with the installation or removal of a switch from an operating cell and subsequent testing. A vacuum switch which has a heavy film deposit of conductor across the insulator body cannot be bolted in place or removed from an operating cell without causing sparking when the terminals are connected or disconnected from the circuit. Such sparking creates a safety hazard within the typical operating environment of the electrolytic cell. The standard technique for determining the integrity and operability of the vacuum switch is to subject the switch to a high voltage across the open contacts to check whether the switch remains evacuated and can withstand the voltage across the contacts. Such a high voltage test cannot be performed if a high leakage current film exists across the insulator body.
The use of electrically floating conductive shields within A.C. vacuum interrupters is well known in the art. Such arcing shields have been used to protect the insulating walls of such evacuated bottles from being coated and/or destroyed by hot evaporated product from the contact areas.